A common problem in designing receiver circuitry is reducing distortion in the receiver's front end. The non-linearity of the receiver components creates spurious waveforms (distortion) that impair the circuit's overall performance. In measurement instrumentation, for example, such spurious waveforms can be confused with or even mask the actual signals being measured.
Any non-ideal device produces a number of distortion products, the most important of which are often second and third harmonics, and second and third order intermodulation distortion. Typically, second order distortion is caused by asymmetry in the output waveform due to non-linear processing of a signal's positive- and negative-going excursions. Third-order distortion is caused by variation in the transfer function of a component with the magnitude of the applied signal.
Third-order distortion can be reduced by driving the device with a smaller signal, or by using a device with a larger dynamic range. In either case, the input signal excursions result in smaller changes in the devices transfer function, thereby resulting in less third-order distortion.
Eliminating second-order distortion, however, has traditionally been more difficult. One way to reduce second-order distortion is to use balanced devices. Balanced devices, however, are never truly balanced. Due to fabrication tolerances, thermal effects and other phenomena, even the most precise (aka expensive) balanced devices have some inherent asymmetry. With this asymmetry comes even-order distortion.
Another approach for controlling second-order distortion is use of a feedback circuit. In such an approach, distortion error is theoretically subtracted out by a feedback signal mixed with the input signal. This method, however, is limited by the gain and non-linearity of the receiver circuit.
As receivers and other electronic circuitry become optimized in other respects, the problem of second order distortion increases in prominence. It is an object of the present invention to address this problem.
In accordance with a preferred embodiment of the invention, even-order distortion products from an electronic device are spectrally redistributed as broadband noise. By spreading the distortion energy over the entire frequency spectrum, the distortion's impact is reduced. Further, conversion of the distortion to broadband noise permits use of known digital signal processing techniques to discern and recover signals below the noise level.
In the preferred embodiment, an auxiliary signal generator generates a pseudo random noise signal that changes between a logic .+-.1. This signal is combined with an input signal in a combiner, such as a multiplier, creating a combined signal having the appearance of broadband noise. This combined signal is then processed by a non-linear device, such as an amplifier or an analog-to-digital converter. After this processing, the signal is synchronously reconstructed by combining the processed signal in a second combiner with the inverse of the auxiliary signal applied to the first combiner. The result is to recover the desired signal while spreading the distortion over the frequency spectrum.
The foregoing and other features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.